1. Field of the Invention
The present invention relates to a continuously variable transmission for a straddle type vehicle.
2. Description of Related Art
V-belt type continuously variable transmissions are widely used in straddle type vehicles such as scooter type motorcycles. A conventional V-belt type continuously variable transmission has paired primary and secondary sheaves having grooves whose widths are variable, and that are respectively provided for a primary shaft to which an output of a drive power source such as an engine is inputted and for a secondary shaft from which an output to a drive wheel is taken. By a V-belt being wound around both sheaves and the groove widths of the respective sheaves being changed by a groove width adjusting mechanism, wind diameters relative to the sheaves are adjusted and a gear ratio between both sheaves is continuously adjusted.
Typically, the primary and secondary sheaves are structured with a fixed flange and a movable flange forming a V-groove therebetween. The movable flanges are movably provided in an axial direction of the primary or secondary shaft. Thus, the gear ratio can be continuously adjusted by moving the movable flanges by the groove width adjusting mechanism.
In one V-belt type continuously variable transmission, the movable flange of the primary sheave is moved by an electrically operated motor. The movable flange is moved in both directions using the moving thrust of the electrically operated motor, i.e., in a direction in which the groove width is narrowed (in a Top gear range) and in another direction in which the groove width is widened (in a Low gear range). Accordingly, the groove width is freely adjustable (for example, see JP-B-3043061).
In a scooter type motorcycle having an electronically controlled V-belt type automatic continuously variable transmission, the gear ratio is automatically changed without a rider's operation (for example, JP-A-Sho 62-175228). Predetermined programs (maps) are executed and the gear ratio is automatically changed based upon information such as vehicle speed, engine rotational speed and acceleration opening. Therefore, the driving operation of the rider is made easier.
In motorcycles having automatic continuously variable transmissions, a speed reduction lever for setting the gear ratio of the continuously variable transmission (for example, see JP-B-2950957), and a switch for compulsorily changing the gear ratio or manually shifting down (engine braking) (see JP-A-Sho 62-175228), are also known. Any gear ratio can thereby be freely and continuously obtained with the rider's intention while also enjoying the advantages of the automatic continuously variable transmission. Application of the engine brake to the driving and previous shift down movements thus can be made, and the slow start of acceleration can be resolved.
In developing a V-belt type continuously variable transmission having a control program (kick down mode) that realizes a shift down movement in accordance with the rider's intention, the inventors analyzed various running conditions, and found a problem that slips can occur between the primary sheave and the V-belt under a deceleration condition when the kick down mode is executed.